This application claims priority from Korean Patent Application No. 2002-69593, filed on Nov. 11, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a hybrid lens combining a diffractive lens with a refractive lens, and more particularly to a hybrid lens with a high numerical aperture used in an optical information recording/reproducing apparatus.
2. Description of the Related Art
The development of an optical information media has led from CDs (Compact disks) having a diameter of 12 cm and a substrate thickness of 1.1 mm to DVDs (Digital Versatile Disks) having a diameter of 12 cm and a substrate thickness of 0.6 mm and is further leading toward blue-ray disc that are thinner than DVDs. Optical apparatuses for recoding and reproducing information from such optical information media have also been researched and developed so that high recording density can be achieved by focusing high optical energy into a small spot. For example, an optical recoding/reproducing apparatus for CDs adopts a light source having a wavelength of 780 nm and a lens having a numerical aperture (NA) of 0.45. Meanwhile, an optical recoding/reproducing apparatus for DVDs adopts a light source having a wavelength of 680 nm and a lens having a numerical aperture of 0.6. The blue-ray disc which is still being researched actively is designed for a light source having a wavelength of about 405 nm and a lens having a numerical aperture of about 0.85. However, manufacturing a lens with such a high numerical aperture is still difficult with a current technology.
FIG. 1 is a diagram showing the relationship between numerical aperture (NA), spot size and depth of focus Δz of a general lens 11.
In FIG. 1, D denotes an aperture of the lens 11, f denotes a focal length and θ denotes an angle of refraction of the light transmitted through the lens. The following Equation 1 defines numerical aperture (NA), wherein n denotes a refractive index of the lens and θ denotes a refractive angel thereof. The following Equation 2 gives the relationship between spot size ω0, numerical aperture NA and wavelength λ, when an incident beam is parallel. The following Equation 3 gives the relationship between depth of focus Δz, numerical aperture NA and wavelength λ.NA=n sin θ  (1)                              ω          0                =                              2            π                    ⁢                      λ                          N              ⁢                                                           ⁢              A                                                          (        2        )                                          Δ          ⁢                                           ⁢          z                =                  λ                      2            ⁢            N            ⁢                                                   ⁢                          A              2                                                          (        3        )            
Spot size should be reduced in order to increase information recording density, and according to Equation 2, wavelength λ should be reduced and numerical aperture NA should be increased in order to reduce spot size ω0. Accordingly, to record and reproduce information on a blue-ray disc, a blue laser with a short wavelength and a lens with a high numerical aperture are required.
However, it is necessary to increase the depth of focus Δz for a stable recording and reproduction, and according to Equation 3, it is necessary to increase the wavelength λ and decrease the numerical aperture (NA) to increase the depth of focus Δz.
To satisfy the Equation 2 and 3, in the case of the blue-ray disc, a cover layer of the disc preferably is manufactured to have about 0.1 mm thickness effectively, which prevents the depth of focus from being shortened, which results from the increase of numerical aperture and decrease of wavelength, thereby increasing the information recording density. However, there is a problem in that increasing the numerical aperture occurs reduction of a tilt allowance range (that is, tilt margin) of the recording surface and light axis. The tilt allowance range can be expanded up to a current DVD level by forming a thinner cover layer of about 0.1 mm. It is possible to maintain allowable ranges of disc deflection and tilt of the blue-ray disc, as well as apparatus assembly errors, etc. at the same level as in DVDs.
The refractive lens used in the optical recording/reproducing medium changes the wavelength of incident laser depending on temperatures. While light with different wavelengths is passed through the refractive lens, the the different wavelengths are focused having different focal lengths in the light axis direction. This is called chromatic aberration.
In the prior art, a thick lens with a large radius of curvature, a low refractive index, and a high Abbe number has been proposed in order to achieve a high numerical aperture while reducing chromatic aberration and dispersion. However, such a lens is difficult to manufacture, and it is large and heavy, making it inappropriate for use in a small and lightweight information recording/reproducing apparatus.